Plug-in connections are known from the prior art (for example, EP 0 655 807 A2, EP 0 854 546 B1 [U.S. Pat. No. 6,022,238], U.S. Pat. No. 5,803,651, DE 195 25 413 A1 [U.S. Pat. No. 5,605,472], DE 196 21 762 A1 [U.S. Pat. No. 5,681,178]) in which a plug and a socket may be joined together to form a plug-in connection. To prevent the plug-in connection from coming apart, locking devices are known which may be brought into at least one locking position after the plug and socket are joined so that the plug and socket cannot be separated during use of the plug-in connection, in particular for automotive applications. To ensure electrical contact for the plug-in connection, the locking device is particularly designed so that it cannot be brought into the locking position until the plug and socket have been completely joined. To ensure this, it is important that the locking device can be easily brought into the locking position. In the locking devices known from the prior art, optional locking elements are known which may be further displaced and which are provided on a housing for the plug or socket, that location being provided with a plug-in region for the locking element in which the locking element may be inserted to assume the locking position. The locking device in addition to the plug-in region and the optional overall system of the locking element on the housing for the plug or socket has the disadvantage that it is largely open, thus permitting contaminants (such as particles generated during the housing manufacturing process, or paints, when the housing for the plug or socket is provided with a coat of paint, or the like) to enter, with the disadvantageous result that the latch element for the locking device in addition to the optionally provided locking element either can no longer be inserted into the plug-in region, or, if it still functions at all, cannot be brought into its important locking position. As a result, the latch element cannot lock with the mating connector, or the locking element may, for example, fall out if it is placed in the plug-in region in a so-called prelocking position, or it may not be possible to completely lock the plug-in connection when the locking element is not in its at least one locking position (also referred to as the final locking position).
Finally, a locking device for a plug or a socket of a plug-in connection is known from DE 103 41 136 which properly performs the important locking function, the locking element designed as a sliding element being first brought into a prelocking position in the plug-in region after manufacture of the plug or socket, and not being moved into its final locking position until the plug and socket of the plug-in connection have been completely joined. Here as well, however, the locking element for the most part is open at the housing for the socket or the plug, thus allowing contaminant particles to penetrate. In addition, this locking element is designed in such a way that it produces an audible locking sound when it is brought from its prelocking position to its final locking position. In an open configuration of the locking element, however, this locking sound radiates into the surrounding space so that it is no longer perceivable, particularly when the movement of the locking element from its prelocking position to its final locking position takes place in a loud production room. Variants of this housing without these additional locking elements are also conceivable. In such variants, for housings for the plug or socket which are already provided with a coat of paint, the latch element may be fixed in such a way that complete joining of the socket and plug is not possible, or may be achieved only with great difficulty. Actuation of the latch element for unlocking such a problematic plug-in connection is then virtually impossible.